This invention relates generally to methods and apparatus for passively shimming a magnet to correct deviations from a desired magnetic field. Methods and apparatus of the present invention are particularly useful in conjunction with magnetic resonance imaging (MRI) apparatus for correcting magnetic field inhomogeneities resulting from manufacturing tolerances. Although the examples cited herein involve MRI apparatus and superconducting magnets, the usefulness of the invention is not limited solely to such apparatus or magnets.
A high uniform magnetic field is useful for using magnetic resonance image (MRI) and nuclear magnetic resonance (NMR) systems as a medical device or a chemical/biological device. At least some popular MRI systems currently available utilize a superconducting magnet that creates a field uniform in a z-direction of about 0.7 Tesla to about 1.5 Tesla in a pre-determined space (i.e., an imaging volume, or volume of interest).
Some types of materials can be made superconducting by placement in an extremely cold environment, such as a cryostat or pressure vessel containing cryogenic material. In the case of a magnet coil, such as those used in MRI apparatus, extreme cold is used to make magnet coils superconducting. In such a state, an initial current produced in the coil continues to flow even after power is removed. Superconducting magnets find wide application in the field of MRI due to their high magnetic field strength.
For proper operation of known MRI apparatus, the magnetic field of the magnet must be uniform in the z-direction to within a specified tolerance, often about 10 ppm. To compensate for inhomogeneities, MRI magnets, including superconducting magnets, are often shimmed utilizing various arrangements of correction coils and/or passive ferromagnetic shim materials.
Open architecture MRI magnets are not immune to field inhomogeneity due to coil deformation and coil misalignment. Known superconducting magnet designs have been directed at minimizing such inhomogeneity during the design stage. To remove inhomogeneity that remains after the manufacturing cycle due to manufacturing tolerances, environmental effects, and/or design restrictions, passive shim systems have been used.
Various shimming methods for MRI magnets are known. At least one known method utilizes a combination of correction coils and passive ferromagnetic shims. The superconducting magnet is adjusted at the factory utilizing these coils and shims to provide a homogeneous magnetic field in the imaging bore of the magnet, which is also referred to herein as a xe2x80x9cvolume of interest.xe2x80x9d Passive shims are positioned between a warm imaging bore and a gradient coil. As a result, in known MRI systems, it is difficult to access and adjust or change passive shims after the gradient coil is installed while minimizing the profile or space occupied by the shim assembly. However, due to magnetic material in the vicinity of the magnet at the installation site, it is frequently necessary to reshim the magnet to provide a specified field homogeneity.
Known passive shimming systems are difficult to adjust on-site. Thus, on-site adjustment has frequently been limited to varying the current through the correction coils. However, it is expensive to provide correction coils, associated circuitry and leads for this purpose.
Referring to FIGS. 1 and 2, at least one known MRI configuration provides passive shimming for a superconducting magnet utilizing a plurality of plastic disks 100 that serve as nonmagnetic pellet holders for pellets 102, 104. Each pellet holder 100 has a number of preformed round holes 106, 108 into which a cylindrical or disk-shaped ferromagnetic pellet 102 or 104, respectively, can be placed. Known configurations utilize two different size holes 106 and 108, each size corresponding to one of two different masses 102 and 104 of ferromagnetic pellet. As many pellets 102 and 104 are inserted into each pellet holder 100 as is required to compensate for inhomogeneities in a magnetic field of the MRI magnet. Pellet holders 100 are held in place in the magnetic field of an MRI magnet utilizing pellet holder mounting hardware 110, wherein a pellet holder 100 is inserted into as many mounting positions 112 of mounting hardware 110 as is required for the desired shimming.
Although effective, known passive shimming systems are subject to various limitations. For example, manufacturing and maintaining two different sizes of shim masses is costly. Moreover, accurate shimming often requires iteration due to manufacturing variations other variables. Convergence of this process can be also slowed as known iterative shimming methods replace one or more pellet-containing holders with holders containing different pellets to compensate for residual magnetic field inhomogeneities. Note, too, that holes 106 and 108 correspond in size, respectively, to the different sizes of pellets 102 and 104. Thus, it is not possible to place a pellet having a large mass into a hole corresponding to a pellet having a smaller mass. This difference in pellet sizes limits the range of compensation that is possible within each pellet holder 110.
Some configurations of the present invention therefore provide a method for shimming a magnetic field of a magnet in a volume of interest, utilizing ferromagnetic pellets for shimming. The pellets are positioned in a nonmagnetic holder having a predetermined number of pellet holes. The method includes measuring the magnetic field of a magnet in a plurality of locations within the volume of interest, determining a nominal passive shimming mass to compensate the measured magnetic field to approximate a desired magnetic field within the volume of interest; and placing a combination of ferromagnetic pellets from a selection of full strength pellets, nearly full strength pellets, and low strength pellets in the pellet holes to approximate the nominal passive shimming mass.
In some configurations, a shimming set having exactly three strengths of ferromagnetic pellets is provided. The shimming set includes full strength pellets, nearly full strength pellets, and low strength pellets. Also included is a nonmagnetic holder configured to hold a fixed number of pellets. A pellet combination selector aid is provided to facilitate selection of a combination of pellets that approximates a desired total mass and that does not exceed the fixed number of pellets.
A method for shimming a magnetic field of a magnet of an MRI apparatus within an imaging volume is provided in some configurations. The magnet is configured to utilize, for shimming, ferromagnetic pellets positioned in a nonmagnetic holder having a predetermined number of pellet holes. The method includes: measuring the magnetic field of the magnet in a plurality of locations within the imaging volume. A nominal passive shimming mass to compensate the measured magnetic field to approximate a desired magnetic field within the imaging volume is determined. A combination of ferromagnetic pellets from a selection of full strength pellets, nearly full strength pellets, and low strength pellets is placed in the pellet holes to approximate the nominal passive shimming mass.
Some configurations of the present invention provide an MRI apparatus that includes a superconducting magnet having a magnetic field shimmed to an approximation of a uniform magnetic field within a volume of interest utilizing exactly three different strengths of ferromagnetic pellets.
Some configurations of the present invention provide a kit of sintered shimming pellets consisting of a plurality of pellets having exactly three different strengths. The provided strengths are full strength, nearly full strength, and low strength. The different strength pellets are of equal size, and pellets of different strengths include different percentage mixtures of iron powder and plastic powder.
Various configurations of the present invention will be seen to reduce shimming quantization errors in comparison to errors usually obtained when utilizing only full strength and low strength pellets for shimming. Also, various configurations of the present invention produce more accurately shimmed magnetic fields and faster convergence when iterative shimming methods are used. In addition, manufacturing and inventory costs are reduced in many configurations as a result of the need to produce and stock only three types of shims.